The present invention relates generally to synthesizing holograms and more particularly to synthesizing holograms digitally from two-dimensional images stored in a memory.
Digital synthesis of holograms from two-dimensional images is used in methods of reproducing three-dimensional images, for example. Respective holograms are computed for two-dimensional digital images representing a three-dimensional object from different viewpoints. These holograms are then combined to produce a hologram of the object which reproduces a three-dimensional image of the object when it is reproduced physically by a spatial light modulator and illuminated by a coherent wave.
There are other applications of digital synthesis of holograms, in particular in telecommunications, radar, X-rays and sonar.
Digital techniques for synthesizing holograms are known in the art. For example, the article by S. Michelin et al. entitled xe2x80x9cFourier-transform computer generated hologram: a variation on the off-axis principlexe2x80x9d published in SPIE Conferences 1994, Practical Holography VIII, pages 249-254, describes a method of simulating the production of an analog hologram. The method consists of applying a Fourier transform to a two-dimensional image, adding a complex field representing a reference optical wave to the Fourier transform obtained in this way, and then extracting the amplitude information contained in the sum of the complex field and the Fourier transform. Applying the Fourier transform to the two-dimensional image digitally simulates the production of a xe2x80x9cdiffractedxe2x80x9d image which results from the diffraction of a fictitious optical wave by the two-dimensional image. The two-dimensional image is also oversampled before the Fourier transform is applied to it. However, the oversampled two-dimensional image obtained in this way is defined by a real intensity distribution which is not always well suited to computing a complex transform such as a Fourier transform.
The present invention aims to provide a method of synthesizing holograms that is more efficient than those of the prior art.
To this end, a method of producing a hologram from a two-dimensional image defined by a real function is characterized in that it comprises the following steps:
transforming the two-dimensional image defined by said real function into a complex two-dimensional image defined by a complex function,
oversampling the complex image,
simulating the production of a diffracted image resulting from the diffraction of an optical wave by the oversampled complex image, and
adding a complex field representing a reference optical wave to the resulting diffracted image in order to produce said hologram.
The method can further comprise the step of encoding values taken by the amplitude of the sum of said complex field and the resulting diffracted image, so that the hologram can be reproduced on a liquid crystal screen or transmitted over a transmission line, for example.
In the present context, a xe2x80x9creal or complex functionxe2x80x9d means a function of two variables, in the form of digital data, and taking real or complex values, respectively. The real function is typically an intensity distribution while the complex function is a distribution of complex numbers each defined by a real amplitude and a real phase.
The step of transforming the given two-dimensional image into a complex image derives from the original two-dimensional image an image which is defined by complex numbers which optimally represent the real optical field and facilitate the computations employed in the simulation step.
The oversampling step increases the number of pixels of the hologram because the computations employed in subsequent steps apply to a greater number of image points. This step can consist of inserting the complex image into a larger image in which the intensity of pixels outside the original complex image is made equal to 0. In this case, implementing the step of oversampling the complex image after the steps of transforming the two-dimensional image into a complex image avoids having to calculate the complex function for points of the oversampled image outside the original complex image.
The transform step typically includes the following steps:
determining amplitude values each depending on the square root of a corresponding value taken by said real function, and
associating a phase with each of said amplitude values so that an amplitude value and a phase value are defined for each point of the complex image.
By averaging the amplitude values of the hologram, associating a phase with each amplitude value avoids peaks of excessively high amplitude in the resulting hologram of the given two-dimensional image.
The simulation step can include computing one of the following complex transforms: Fourier transform, Walsh transform, Hankel transform, orthogonal polynomial transform, Hadamar transform, Karhunen-Loeve transform, multiresolution discrete wavelet transform, adaptive wavelet transform and a transform which is a composite of at least two of the above transforms.
The simulation step advantageously consists of computing a convolutional product, associated with the oversampled complex image, of two components, by applying the transform which is the inverse of said complex transform to the product of the respective complex transforms of said two components.
Until now, the skilled person has regarded the Fourier transform, which is widely used in optics, as the best possible transform for calculating a convolutional product of this kind. However, experiments conducted by the present inventors have shown that using one of the complex transforms mentioned above other than the Fourier transform produces, for a two-dimensional image, a resultant hologram of much better quality, i.e. which, when it is reproduced physically and illuminated by a coherent source, produces an image associated with the two-dimensional image that is finer than those generally produced by prior art systems.
According to another aspect of the invention, a method of producing a hologram from a two-dimensional image defined by a real function is characterized in that it comprises the following steps:
oversampling the two-dimensional image,
transforming the oversampled two-dimensional image into a complex two-dimensional image defined by a complex function,
simulating the production of a diffracted image resulting from the diffraction of an optical wave by the oversampled complex image, and
adding a complex field representing a reference optical wave to the resulting diffracted image to produce said hologram.
The invention also provides a system for producing a hologram from a two-dimensional image defined by a real function, characterized in that it comprises:
transform means for transforming the two-dimensional image defined by said real function into a complex two-dimensional image defined by a complex function,
means for oversampling the complex image,
simulator means for simulating the production of a diffracted image resulting from the diffraction of an optical wave by the oversampled complex image, and
means for adding a complex field representing a reference optical wave to the resulting diffracted image to produce said hologram.
The system can further comprise means for encoding values taken by the amplitude of the sum of said complex field and the diffracted image.
The transform means can comprise means for determining amplitude values each depending on the square root of the corresponding value taken by said real function and means for associating a phase with each of said amplitude values so that an amplitude value and a phase value are defined for each point of the complex image.
The simulator means can comprise means for computing one of the following complex transforms: Fourier transform, Walsh transform, Hankel transform, orthogonal polynomial transform, Hadamar transform, Karhunen-Loeve transform, multiresolution discrete wavelet transform, adaptive wavelet transform and a transform which is a composite of at least two of the above transforms.
The simulator means advantageously comprise means for computing a convolutional product, associated with the oversampled complex image, of two components, by applying the transform which is the inverse of said complex transform to the product of the respective complex transforms of said two components.
According to another aspect of the invention, a system for producing a hologram from a two-dimensional image defined by a real function is characterized in that it comprises:
means for oversampling the two-dimensional image,
transform means for transforming the oversampled two-dimensional image into a complex two-dimensional image defined by a complex function,
simulator means for simulating the production of a diffracted image resulting from the diffraction of an optical wave by the oversampled complex image, and
means for adding a complex field representing a reference optical wave to the resulting diffracted image to produce said hologram.